The invention is based on an arrangement having a microprocessor and a switched-mode power supply with a control loop, the switched-mode power supply having a normal mode and a low-power mode, e.g. a standby mode, with a burst mode. Arrangements of this type are used in television sets or video recorders, for example.
In the low-power mode, switched-mode power supplies frequently use a so-called burst mode, in which the switching transistor is turned off completely at a low clock frequency, e.g. 100 Hz, during an off-phase. During the on-phase of the burst mode, the switched-mode power supply operates at its normal switching frequency, at which there is active control, e.g. 16 kHz. This means that the burst mode ensures that the switched-mode power supply transfers power to the secondary side only during the short interval of the on-phase, in which the said switched-mode power supply is able to operate at a high switching frequency; as a result of this, a switched-mode power supply, in particular an isolating-transformer switched-mode power supply, can be used to achieve very low standby powers. Switched-mode power supplies having a burst mode are disclosed in EP-A 0 386 989 and DE-A-195 18 863, for example.
This burst mode is produced either by a special switched-mode power supply IC or by an additional circuit arrangement on the primary side of the switched-mode power supply.
The invention is based on the object of specifying an arrangement which has a very reliable burst mode and, in addition, reduces the circuit complexity.
This object is achieved by the invention indicated in claim 1. Advantageous developments of the invention are indicated in the subclaims.
According to the invention, a switched-mode power supply having a control loop and having a normal mode and a low-power mode with a burst mode is connected to a microprocessor connection by means of which the said microprocessor controls the burst mode of the switched-mode power supply. Since devices such as television sets or video recorders usually already have a microprocessor anyway, the circuit complexity for the burst mode is very low. This is particularly true in a switched-mode power supply with mains isolation if the control loop of the switched-mode power supply monitors a secondary output voltage, so that the output of the microprocessor can be connected directly to the control loop via one or more resistors or possibly a transistor stage. In battery-operated devices without mains isolation, the arrangement can be designed in the same way.
This means that the microprocessor is able to control the clock frequency and the duty cycle of the burst mode directly without an analogue/digital converter, for example using TTL logic.
A fixed duty cycle can be defined for the burst mode by the microprocessor, which provides steady burst-mode operation even with very low standby powers, such as 2 watts. By changing the clock frequency and the duty cycle, the switched-mode power supply can be matched quickly to changes in conditions. The burst mode can be started in a controlled manner whenever a user switches the appropriate device to the low-power mode, and is not produced indirectly, as is presently the case in a television set, for example, where turning off the deflection and the video stages, which causes the output voltages of the switched-mode power supply to rise, initiates the low-power-mode.
In the low-power mode, the microprocessor-controlled burst mode can also reduce the output voltages considerably, for example to 50%. In a television set, this can produce a soft picture collapse. For this type of use, when a user switches the television set to low-power mode, the microprocessor firstly reduces the system voltage and only then, after a delay, turns off the deflection and the video circuit. Without this procedure, the picture tube of the television set would light up briefly. When switching from low-power mode to normal mode, the deflection and the video stage are switched on after a delay, and only after the output voltages of the switched-mode power supply have stabilized. This means that faults in the deflection cannot occur when it is turned on, because, when it is turned on, its power consumption loads the output voltages of the switched mode power supply significantly, so that voltage fluctuations can occur.
In addition, the microprocessor-controlled burst mode reliably handles an overload, caused by a short circuit, for example. In the previous burst mode, the switched-mode power supply would change over to the normal mode in the event of a short circuit, because the higher power requirement causes it to assume that the television set has been switched to normal mode. In the microprocessor-controlled burst mode, on the other hand, the burst mode is defined to be fixed, and changeover is possible only as a result of a user command via the microprocessor. It is likewise impossible for the switched-mode power supply to change to the burst mode unintentionally, for example if the power consumption in the normal mode is very low for a short time. Since the burst mode is initiated indirectly in previous designs, never directly by a user, the logic decision for the switched-mode power supply to change to the required low-power mode is never as reliable as when there is direct input via the microprocessor.